Yahoo! launched two own applications for Android, Yahoo! Messenger and Yahoo! Mail. I've installed them both, and I must tell you that I am disappointed by both. The messenger is completely useless. It connects to Yahoo!, but it doesn't display any contact as online. All my contacts are seen as offline, even if I currently have about 45 of them online (and visible, of course).
Regarding to the mail, it works, but it does it really slow. Too slow for a mail client, in my opinion. Viewing through the content of the mails is fine, but the time from when you select another mail and you view it, is simply too much. On a 1 GHz processor and 512 MB of ram, with Android 2.2 Froyo installed. OK, if it works like this on Nexus One, how does it work on slower devices?!
Have you installed these two programs? How are they working (if they are...) on your device?